Wiring most bicycles is fairly basic. If everything is wired correctly and it still don’t work you need to troubleshoot individual components. The first thing to check is ground. Ensure all contacts are clean and free of corrosion. With switches you can use a test light to see if power is running through when pushed or switched. Even though most horns weren’t designed to be serviced they can be carefully taken apart and cleaned. Most horns also have an adjustment. A couple of things to try before doing this are smacking it hard against a solid surface to see if it loosens. You can also momentarily hit it with a 9v battery. If both of these fail carefully pry the retaining tangs to get at the disc and clean it.